Method and apparatus for overshoot and undershoot errors correction in analog low dropout regulators

ABSTRACT

The present invention provides a method and apparatus for dynamically correcting overshoot and undershoot errors in an analog integrated circuit by improving the reaction time (Δt) of the analog integrated circuit. Equivalently, an error correction circuit is disclosed present invention is only activated to reduce overshoot and undershoot errors by increasing the bandwidth of the integrated circuit when either undershoot or overshoot errors are detected.

FIELD OF THE INVENTION

The present invention relates generally to the field of analogintegrated circuits. More specifically, the present invention relates toanalog voltage regulators.

BACKGROUND ART

Voltage regulators that provide clean output signals are essential tothe performance of analog integrated circuits. This is especially truewhen analog integrated circuits are used in portable electronic devicessuch as digital cameras, cellular phones, and laptop computers thatrequire low quiescent currents and a low supply voltage from 1.9 voltsto 3.3 volts. Transient responses of these analog integrated circuitscan cause irreversible failure and often shorten the lifetime of theseportable devices. Low drop-out voltage regulator circuits are widelyused because they provide a stable, low noise and well specified DCoutput voltage for integrated circuits. However, low drop-out voltageregulator circuits are vulnerable to transient overshoot and undershooterrors that are caused by the switching on and off of a downstream loaddevice.

With reference to the schematic diagram of FIG. 1A, a prior art lowdrop-out voltage regulator circuit 100 connected to a downstream loaddevice represented by a load current 112 is shown that includes an erroramplifier 101, a pass device 102, and a reference network including afirst resistor (R₁) 103 and a second resistor (R₂) 104. Low drop-outvoltage regulator circuit 100 produces an output voltage (V_(OUT))independent of the input voltage (V_(IN)) and proportional to areference voltage (V_(REF)). FIG. 1B shows a waveform 119 representingload current 112 and a waveform 120 representing the output signal(V_(OUT)). When load current 112 is turned on, the current increasesfrom 0 mA to 500 mA which is represented by a rising edge 119U ofwaveform 119. In response, the voltage on capacitor 105 decreases butlow drop-out voltage regulator circuit 100 does not react fast enough tocompensate for the sudden drop in the output voltage (V_(OUT)). As aresult, an undershoot error 121 exists in the transient response of theoutput signal (V_(OUT)). The magnitude of undershoot error 121 isΔV⁻=I_(L). Δt/C (Equation 1), where C is the capacitance of capacitor105.

Continuing with FIG. 1B, when load current 112 is turned off, thecurrent decreases from 500 mA to 0 mA which is represented by a fallingedge 119D of waveform 119. In response, the voltage of capacitor 105increases but low drop-out voltage regulator circuit 100 does not reactfast enough to stop compensating for the output voltage (V_(OUT)) whichis represented by a waveform 120. This results in an overshoot error 122in output waveform 120. The magnitude of overshoot error 122 isΔV⁺=I_(L). Δt/C (Equation 2). Typically, in order to solve undershooterror 121 and overshoot error 122 in low drop-out voltage regulatorcircuit 100, a large capacitor 105 between 10 μF and 100 μF is connectedto the output terminal 109 and an electrical ground 111. Largecapacitance C in the denominator of Equation 1 and Equation 2 reducesthe magnitude of ΔV⁻ and ΔV⁺. However, large capacitor 105 requires asignificant amount of board area as well as increases in manufacturingcosts. In addition, large capacitor 105 tends to slow the response timeof low drop-out voltage regulator circuit 100. On the other hand,reducing the value of capacitor 105 speeds up the reaction time but maycause instability and an increase the overshoot (equation 2) in lowdrop-out voltage regulator circuit 100. Therefore, changing thecapacitance (C) of capacitor 105 is not feasible to handle overshoot andundershoot problems. Another method is to reduce the reaction time (Δt)in equations 1 and 2 by using very fast error amplifier 101. However,fast error amplifiers require expensive process technology and complexcircuit design. Thus, additional circuitry is needed to solve thetransient overshoot and undershoot errors in low drop-out voltageregulator 100.

Many prior arts have attempted to provide additional circuitry to solvethe overshoot and undershoot errors problems in low drop-out voltageregulator 100. In one prior art, an output stage compensation circuitelectrically coupled between error amplifier 101 and pass device 102 isdisclosed. Inside the prior art's output stage compensation circuit, oneor more segmented sense devices are configured to provide pole-zerocompensation to low drop-out voltage regulator circuit 100 based onoutput current. Each of the segmented sense devices is configured tocompensate a suitable range of output current and to multiply the effectof associated compensation capacitors. As a result, the output stage ofcompensation circuit of the prior art provides a stable output voltage(V_(OUT)) which is not dependent upon the output current and thecapacitance requirements. However, the disclosed output stagecompensation circuit does not address overshoot error and undershooterror problems. Furthermore, the prior art circuits do not provide asolution to low quiescent current requirements and economy in siliconarea.

Accordingly, there are unmet needs for a method and an overshoot andundershoot correction circuit that enable low drop-out voltage regulatorcircuits to achieve fast reaction time so as to solve for the overshooterror and undershoot error problems. In addition, there is an unmet needfor an overshoot and undershoot correction circuit that does not consumea large amount of quiescent currents and occupy large circuit boardarea. The present invention meets the above needs.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for correctingovershoot and undershoot errors by improving the reaction time (Δt) ofan analog integrated circuit, or, equivalently, dynamically increasingthe bandwidth of the analog integrated circuit is when the undershootand undershoot errors are detected. In other words, the presentinvention provides an apparatus that is only activated when overshootand undershoot errors are detected to increase the speed (reducing Δt)of the loop bandwidth of an analog integrated circuit.

An error correction circuit for an analog integrated circuit isdisclosed that includes an undershoot correction circuit and a separateovershoot correction circuit. When an undershoot error is detected, theundershoot correction circuit is turned on and causes the analogintegrated circuit to immediately start compensating for the undershooterrors. And when an overshoot error is detected, the overshootcorrection circuit is turned on, which turns off the pass transistor andthus stops providing current that charges the output capacitor.

A method of correcting overshoot and undershoot errors in an analogcircuit is also disclosed that includes the steps of sensing forundershoot and overshoot errors in an analog integrated circuit using anerror correction circuit that includes an undershoot correction circuitand an overshoot correction circuit, whenever an undershoot error or anovershoot error is detected, activating either undershoot or overshootcorrection circuit so as to generate a corrective signal, and using thecorrective signal to cause the analog integrated circuit to react faster(decreasing Δt, increasing the loop bandwidth) and thus substantiallyreducing the undershoot and overshoot errors. One of the advantages ofthe present invention is that the overshoot and undershoot correctioncircuits consume minimal quiescent power because they are in the offmode during normal operating condition.

A low drop-out voltage regulator that uses a low input offset erroramplifier and the error correction circuit is also disclosed. The lowinput offset error amplifier has a fast reaction time (Δt), a low DCoffset input voltage, an improved noise and distortion performance sothat it can effectively work with the error correction circuit tofurther solve the overshoot and undershoot problems in low drop-outvoltage regulator circuits. The low input offset error amplifier of thepresent invention includes an input differential stage, a gain stage,and an output stage. The gain stage, electrically coupled between theinput differential stage and the output stage, is operable to providethe gain for the error amplifier and to prevent the offset currents fromreflecting back to the input differential stage by using common-modefeedback scheme.

These and other advantages of the present invention will no doubt becomeobvious to those of ordinary skill in the art after having read thefollowing detailed description of the preferred embodiments, which areillustrated in the various drawing Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

FIG. 1A illustrates a schematic diagram of a prior art low drop-outvoltage regulator that is vulnerable to overshoot and undershootproblems.

FIG. 1B illustrates waveforms of the load current and the output signalof the prior art low drop-out voltage regulator circuit of FIG. 1A.

FIG. 2 illustrates a block diagram of an analog integrated circuit (IC)electrically connected to an error correction circuit that includes anovershoot correction circuit and a separate undershoot correctioncircuit in accordance with an embodiment of the present invention.

FIG. 3 illustrates a schematic diagram of a low drop-out voltageregulator circuit that uses the error correction circuit of FIG. 2 toreduce undershoot error and overshoot error in accordance with anembodiment of the present invention.

FIG. 4 illustrates a complete schematic diagram of an undershootcorrection circuit in accordance with an embodiment of the presentinvention.

FIG. 5 illustrates a complete schematic diagram of an overshootcorrection circuit in accordance with an embodiment of the presentinvention.

FIG. 6 illustrates a complete schematic diagram of a low drop-outvoltage regulator integrated circuit with the error correction circuitin accordance with an embodiment of the present invention.

FIG. 7 illustrates a complete schematic diagram of a low input offseterror amplifier used in a low drop-out voltage regulator circuit toreduce the overshoot error and the undershoot error in a low drop-outvoltage regulator in accordance with an embodiment of the presentinvention.

FIG. 8 illustrates a flow chart of a method of correcting overshoot andundershoot errors in analog integrated circuits in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction with thepreferred embodiments, it will be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents, which may be included within the spirit and scope of theinvention as defined by the appended claims. Furthermore, in thefollowing detailed description of the present invention, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be obvious toone of ordinary skill in the art that the present invention may bepracticed without these specific details. In other instances, well-knownmethods, procedures, components, and circuits have not been described indetail so as not to unnecessarily obscure aspects of the presentinvention.

Referring to FIG. 2, an error correction circuit electrically coupled toaddress the overshoot and undershoot errors in an analog integratedcircuit (IC) system in accordance with an embodiment of the presentinvention is illustrated. An analog integrated system 200 includes ananalog integrated circuit 210 electrically connected to an errorcorrection circuit 220. According to an embodiment of the presentinvention, error correction circuit 220 further includes an undershootcorrection circuit 201 and a separate overshoot correction circuit 202.Undershoot correction circuit 201 is used to address undershoot errorsand overshoot correction circuit 202 is used to address overshooterrors.

Structurally, analog integrated circuit 210 includes an input terminal207 that receives an input voltage (V_(IN)) and an output terminal 209that produces an output voltage (V_(OUT)). Output terminal 209 is alsoconnected to a downstream load represented by load current 112 connectedin series to electrical ground 111. It is noted that within the scope ofthe present invention, analog integrated circuit 210 can be but notlimited to low drop-out voltage regulators, analog amplifiers, analogoperational-amplifiers (op-amps), analog to digital converters (ADC), orany analog integrated circuits that are vulnerable to the transientundershoot and undershoot errors. Furthermore, the terms undershooterror and overshoot error mean a transient response in the output signal(V_(OUT)) due to square waveform 119 of load current 112 as illustratedin FIG. 1B. However, it is noted that, any unwanted transient signals inthe output voltage (V_(OUT)) are covered within the meanings of theterms error in the present invention.

Referring again to FIG. 2, undershoot correction circuit 201 is separateand different from overshoot correction circuit 202. Undershootcorrection circuit 201 and overshoot correction circuit 202 areelectrically coupled to analog integrated circuit 210 to separatelyaddress undershoot and overshoot errors in output voltage (V_(OUT)).Undershoot correction circuit 201 has an input terminal 432 and anoutput terminal 433. Input terminal 432 is electrically connected toreceive output voltage (V_(OUT)) of analog integrated circuit 210. Assoon as an undershoot error in the output signal (V_(OUT)) is detected,undershoot correction circuit 201 is activated and produces a correctivesignal at output terminal 433, immediately causing analog integratedcircuit 210 to compensate for the output signal (V_(OUT)). In otherwords, undershoot error circuit 201 reduces the reaction time Δt ofanalog integrated circuit 210. More particularly, referring again toEquation 1, ΔV⁻=I_(L). Δt/C, given C and I_(L) unchanged, if thereaction time Δt is significantly reduced, the magnitude of theundershoot error ΔV⁻ will also be significantly reduced. This means thatthe bandwidth of analog integrated circuit 210 is dynamically increasedwhen needed when the undershoot and undershoot errors are detected.

Over all, overshoot correction circuit 202 operates in the sameprinciple as undershoot correction circuit 201. However, overshootcorrection circuit 202 also creates an electrical path to compensate theovershoot error. Overshoot correction circuit 202 has an input terminal501 and an output terminal 528. Input terminal 501 is electricallycoupled to receive the output voltage (V_(OUT)) while output terminal528 is electrically connected to analog integrated circuit 210. As soonas an overshoot error is detected, overshoot correction circuit 202issues a corrective signal at output terminal 528, causing analogintegrated circuit 210 to immediately compensate the overshoot error inthe output voltage (V_(OUT)). At the same time, overshoot correctioncircuit 202 creates an electrical path so that the excess voltage of theovershoot error can be reduced. In other words, overshoot error circuit202 reduces the reaction time Δt of analog integrated circuit 210 ordynamically increases the overall system bandwidth. More particularly,referring again to Equation 2, ΔV⁺=I_(L). Δt/C, given C and I_(L)remained the same, if the reaction time Δt is significantly reduced, themagnitude of the overshoot error ΔV⁺ will also be significantly reduced.

In conclusion, analog integrated system 200 with separate undershootcorrection circuit 201 and overshoot correction circuits 202 can produceclean output signal (V_(OUT)) without undershoot and overshoot errors.Furthermore, since overshoot correction circuit 202 and undershootcorrection circuit 201 are only activated when overshoot error andundershoot error are detected, error correction circuit 220 efficientlyaddresses and solves undershoot and overshoot errors in the outputsignal (V_(OUT)) without consuming a large amount of quiescent current.Low quiescent current translates to low power consumption.

Now referring to FIG. 3, a schematic diagram of a low drop-out voltageregulator 300 that includes overshoot correction circuit 202 andundershoot correction circuit 201 of the present invention to reduce theovershoot and undershoot errors is illustrated. Input terminal 432 ofundershoot correction circuit 201 is electrically connected to outputterminal 209 at node V_(B) to detect undershoot errors in output signal(V_(OUT)). Output terminal 433 of undershoot correction circuit 201 iselectrically coupled to an output stage (not shown) of error amplifier101. As soon as undershoot error occurs in the output signal (V_(OUT)),undershoot correction circuit 201 issues a corrective signal to causeerror amplifier 101 to react right away and thus improving the reactiontime Δt of low drop-out voltage regulator circuit 300.

Continuing with FIG. 3, input terminal 501 of overshoot correctioncircuit 202 is electrically connected to output terminal 209 at nodeV_(B) to sense the output voltage (V_(OUT)) to detect any overshootthereon. Output terminal 528 is electrically connected to the gate ofpass transistor 102 at node V_(A). In a normal condition where there isno overshoot error, the voltage at the gate of pass transistor 102 isgreater than the output voltage (V_(OUT)). However, when the outputcurrent on output terminal 209 is suddenly decreased from 500 mA to 0mA, pass transistor 102 is not immediately turned off, causing outputvoltage (V_(OUT)) at node V_(B) to increase. This results in overshooterror in the transient response of output signal (V_(OUT)) as shown inFIG. 1B. As soon as the overshoot error is detected, overshootcorrection circuit 202 is activated and produces a corrective signal toprevent pass transistor 102 from further supplying current to capacitor(C) 105. Therefore, the overshoot error is eliminated.

Referring next to FIG. 4, a detailed schematic diagram of undershootcorrection circuit 201 in accordance with an embodiment of the presentinvention is illustrated. Undershoot correction circuit 201 operates asdescribed above and includes an input stage circuit 410 electricallycoupled to a differential stage circuit 420, and an output stage circuit430. In one embodiment of the present invention, input stage circuit 410includes a buffer 411 electrically connected in series to a capacitor412. The input of buffer 411 is electrically connected to input terminal432. The output terminal of buffer 411 is coupled to differential stagecircuit 420 via capacitor 412. In one embodiment, buffer 411 is a fastAB class buffer. In one embodiment, buffer 411 is a simple buffer usedfor buffering output signal (V_(OUT)). Any voltage drop caused byundershoot error in output signal (V_(OUT)) in buffer 411 will causecapacitor 412 to discharge. Differential stage circuit 420 is a fastamplifier that includes a first NMOS transistor 421 and a second NMOStransistor 422. The gate of first NMOS transistors 421 is electricallyconnected to the gate of second NMOS transistor 422. The source of firstNMOS transistor 421 is electrically connected to a first resistor 425and to capacitor 412. The source of second NMOS transistor 422 iselectrically connected to a second resistor 426. The second terminals ofboth first resistor 425 and second resistor 426 are electricallyconnected to electrical ground 111. First NMOS transistor 421 and secondNMOS transistor 422 are biased by a biasing circuitry that includes afirst current source 428, a first PMOS transistor 423, a second PMOStransistor 424, and a third NMOS transistor 427. The gate of first PMOStransistor 423 is electrically connected to the gate and the drain ofsecond PMOS transistor 424. The drain of first PMOS transistor 423 is,in turn, connected to the drain of first NMOS transistor 421 and tooutput stage circuit 430. The drain of second PMOS transistor 424 iselectrically connected to the drain second NMOS transistor 422 and tothe gates of first PMOS transistor 423 and second PMOS transistor 424.The source of first NMOS transistor 423 is electrically connected to thesource of second PMOS transistor 424 and to a supply voltage (V_(CC))110. The first terminal of current source 428 is electrically connectedto supply voltage (V_(CC)) 110. The other terminal of current source 428is electrically connected to the drain and the gate of third NMOStransistor 427. The gate of third NMOS transistor 427 is electricallyconnected to the gates of first NMOS transistor 421 and second NMOStransistor 422. The source of third NMOS transistor 427 is electricallyconnected to a third resistor 429. The other terminal of resistor 429 iselectrically connected to electrical ground 111. Output stage 430includes a PMOS transistor 431. The gate of PMOS 431 is electricallyconnected to the drains of first PMOS transistor 423 and first NMOStransistor 421. The source of PMOS transistor 431 is electricallyconnected to supply voltage (V_(CC)) 110. Finally, the drain of PMOStransistor 431 forms output terminal 433. In one embodiment, resistors425, 426, and 429 are 100 KΩ, first current source 428 is 4 μA, and thesupply voltage (V_(CC)) 110 is 2.5 volts. In one embodiment of thepresent invention, first current source 428 is a resistor (not shown)electrically connected between supply voltage (V_(CC)) 110 and thedrain-gate of third NMOS transistor 427. In one embodiment, firstcurrent source 428 is an active current mirror that produces a constantbiasing current.

Referring now to FIG. 5, the schematic diagram of overshoot correctioncircuit 202 in accordance with an embodiment of the present invention isnow shown in details. Overshoot correction circuit 202 includes anoutput stage circuit 510 electrically coupled to a differential stagecircuit 520. In one embodiment of the present invention, output stage510 includes an NMOS transistor 511. Differential stage 520 includes afirst pnp bipolar junction transistor 521 and a second pnp bipolarjunction transistor 522 which are biased by a biasing circuitry thatinclude a first npn bipolar junction transistor 523, a second npnbipolar junction transistor 524, a third pnp bipolar junction transistor525, and a constant current source 526. More particularly, the emitterof first pnp bipolar junction transistor 521 is electrically connectedto the first terminal of a resistor 527. The other terminal of resistor527 forms output terminal 528. The base of first pnp bipolar junctiontransistor 521 and the base of second pnp bipolar junction transistor522 are electrically connected together. The collector of first pnpbipolar junction transistor 521 is electrically connected to thecollector and the base of first npn bipolar junction transistor 523;while the collector of second pnp bipolar junction transistor 522 iselectrically connected to the collector of second npn bipolar junctiontransistor 524. The bases of both first npn bipolar junction transistor523 and second npn bipolar junction transistor 524 are connectedtogether. The emitters of first npn bipolar junction transistor 523 andsecond npn bipolar junction transistor 524 are connected together and toelectrical ground 111. The base of first pnp bipolar junctiontransistors 521 and the base of second pnp bipolar junction transistor522 are electrically connected to the base of third pnp bipolar junctiontransistor 525. The collector of third pnp bipolar 525 is electricallyconnected to its base and to constant current source 526. The other sideof constant current source 526 is electrically connected to electricalground 111. In one embodiment of the present invention, constant currentsource 526 has a constant current value of 10 μA. The emitter of thirdpnp bipolar junction transistor 525 is electrically connected to theemitter of first npn bipolar junction transistor 521 and to the firstterminal of resistor 527. The emitter of second pnp bipolar junctiontransistor 522 is electrically connected to the drain terminal of NMOStransistor 511 of output stage circuit 510. The gate of NMOS transistor511 is electrically connected to the collector junction of second pnpbipolar junction transistor 522 and second npn bipolar junctiontransistor 524. The source of NMOS transistor 511 is electricallyconnected to electrical ground 111. The drain of NMOS transistor 511 iselectrically connected to the emitter of second pnp bipolar junctiontransistor 522 and to input terminal 501.

Now referring to FIG. 6, a complete schematic diagram of low drop-outvoltage regulator (LDO) circuit 600 that has undershoot correctioncircuit 201 and overshoot correction circuit 202 in accordance with anembodiment of the present invention is shown. In an embodiment of thepresent invention, analog integrated system 200 of FIG. 2 is a lowdrop-out voltage regulator circuit 600 that includes error amplifier601, pass device 610, overshoot correction circuit 202, and undershootcorrection circuit 201, all fabricated on the same chip. In oneembodiment of the present invention, error amplifier 601 is a prior arterror amplifier 101. In another embodiment, error amplifier 601 is a lowinput offset error amplifier 700 which will be described later. Erroramplifier 601 includes a first input terminal (positive input terminal)602, a second input terminal (negative input terminal) 603, and anoutput terminal 604. First input terminal 602 receives a referencevoltage (V_(REF)) which can be generated by a reference voltage source(not shown). Second input terminal 603 receives a sampled and scaledsignal (V_(SENSE)) from pass device 610. In one embodiment of thepresent invention, pass device 610 is an NMOS transistor 611. The drainof NMOS transistor 611 is connected to an input terminal 612 of lowdrop-out voltage regulator 600 where an unregulated input signal(V_(IN)) is received. The gate of NMOS transistor 611 is connected inseries to output terminal 604 of error amplifier 601. The source of NMOStransistor 611 is connected to a feedback circuit 620 that furtherincludes a first sampling resistor (R₁) 621, a second sampling resistor(R₂) 622. The source of NMOS transistor 611 is also connected tocapacitor 623 having a capacitance C. The other terminal of capacitor623 is connected in series with ESR resistor 624. The other terminal ofESR resistor 624 is electrically connected to electrical ground 111. ESRresistor 624 is the “Equivalent Series Resistance” of capacitor 623.

More particularly, the source of NMOS transistor 611 is connected tofirst sampling resistor 621. First sampling resistor (R₁) 621 isconnected in series with second sampling resistor (R₂) 622 and to secondinput terminal 603 of error amplifier 601. The other terminal of secondsampling resistor (R₂) 622 is electrically connected to electricalground 111.

Continuing with the description of low drop-out voltage regulatorcircuit 600 of FIG. 6, input terminal 432 of undershoot correctioncircuit 201 is electrically connected to output terminal 630 of NMOStransistor 611 at node V_(B) to sense the output voltage (V_(OUT)) forany undershoot error. The output signal (V_(OUT)) is buffered by buffer410 of undershoot correction circuit 201. The output terminal 433 ofundershoot correction circuit 201 is electrically coupled to an outputstage (not shown) of error amplifier 601 so that when an undershooterror occurs, undershoot correction circuit 201 cause error amplifier601 to issue a corrective signal to NMOS transistor 611. Input terminal501 of overshoot correction circuit 202 is electrically connected tooutput terminal 630 to sense output voltage (V_(OUT)) for any overshooterror at node V_(B). The output terminal 528 is connected to control thegate of NMOS transistor 611 at node V_(A). Output terminal 630 of lowdrop-out voltage regulator circuit 600 is electrically connected to aload represented by load current 112 having a current I_(L) andcapacitor 623.

With reference to the description of FIG. 6 above, the principle ofoperation of low drop-out voltage regulator circuit 600 can be describedas follows. Error amplifier 601 compares voltage reference (V_(REF))with sampled and scaled output voltage (V_(SENSE)) and generates acorrective signal to regulate the voltage drop across NMOS transistor611 such that condition V_(REF)=V_(SENSE) is fulfilled. When outputsignal (V_(OUT)) does not have any undershoot error or overshoot error,undershoot correction circuit 201 and overshoot correction circuit 202are in a cutoff state. However, when undershoot error is detected atinput terminal 432, undershoot correction circuit 201 is activated andcauses error amplifier 601 to react faster to compensate for theundershoot error in output signal (V_(OUT)). More particularly, in anormal operating condition without undershoot error, voltage level atinput terminal 432 is at DC value of output signal (V_(OUT)). Thisvoltage level is buffered at buffer 411. First NMOS transistor 421 is incutoff mode because second NMOS transistor 422 is turned on and drawingmore current from gate to electrical ground 111. This is because secondNMOS transistor 422 is physically larger than first NMOS transistor 421.As a result, first NMOS transistor 421 is turned off because its gatevoltage is pulled to electrical ground 111 by second NMOS transistor422. Under normal operating conditions, active bias current sourceconfigured by first PMOS transistor 423 and second PMOS transistor 424is biased so that PMOS transistor 431 in the cutoff state, no currentflowing out of output terminal 433. Thus, under normal condition whenthere is no undershoot error at output terminal 630, undershootcorrection circuit 201 is in high impedance state, i.e., no currentflowing out at output terminal 433.

Referring again to FIG. 6, when an undershoot error occurs, buffer 411quickly senses the voltage drop at input terminal 432. As a result, acorresponding voltage drop occurs at the other side of capacitor 412. Avoltage drop is observed across second resistor 425 of the source offirst NMOS transistor 421. In response to this voltage drop at thesource, the voltage gap between gate and source widens and second NMOStransistor 422 is turned on, pulling the voltage at the gate of PMOStransistor 431 toward electrical ground 111. PMOS transistor 431 is nowturned on and raises the voltage at output terminal 433 to supplyvoltage (V_(CC)) 110. The increase in the voltage at output terminal 433causes error amplifier 601 to react and start compensating for theundershoot error. Thus, when undershoot error occurs at output terminal630, undershoot correction circuit 201 is in low impedance state andcauses current to flow from output terminal 433 to error amplifier 601.

Continuing with FIG. 6, in a normal operating condition where there isno overshoot error detected at output terminal 630, overshoot correctioncircuit 202 is in a cutoff condition. More particularly, when overshooterror is not detected, the voltage at the gate of NMOS transistor 611 atnode V_(A) is higher than the voltage at output terminal 630. As such,in overshoot correction circuit 202, the voltage at output terminal 528is higher than the voltage at input terminal 501. Equivalently, thevoltage at the emitter of pnp bipolar junction transistor 521 is higherthan that of pnp bipolar junction transistor 522. In such condition,first pnp bipolar junction transistor 521 conducts more than second pnpbipolar junction transistor 522. Thus, more current flows on thecollector of diode-connected npn bipolar junction transistor 524. As aresult, current mirror formed by npn bipolar junction transistors 523and 524 will cause npn bipolar junction transistor 524 to turn on,pulling the gate of NMOS transistor 511 toward electrical ground 111. Innormal operating conditions, NMOS transistor 511 is in a cutoff mode;and therefore, overshoot correction circuit 202 is in the high impedancestate.

On the other hand, as soon as overshoot error is detected at inputterminal 501, overshoot correction circuit 202 is activated to dissipatethe overshoot error. More particularly, when the normal operatingcondition is reversed and overshoot error is detected, the increase involtage of overshoot error 122 as shown in FIG. 1B causes the voltage atinput terminal 501 to be higher than the voltage at output terminal 528.In response, second pnp bipolar junction transistor 522 is turned onwhile first pnp bipolar junction transistor 521 is turned off. Thiscauses current to flow on the collector of second pnp bipolar junctiontransistor 522, and pulling its output to a high state. Now NMOStransistor 511 is turned on, dumping the excess voltage of the overshooterror to electrical ground 111. In the meantime, first pnp bipolarjunction transistor 521 is turned off.

Now referring to FIG. 7, in an embodiment of the present invention,error amplifier 601 used in low drop-out voltage regulator 600 comprisesa low input offset error amplifier 700. Low input offset error amplifier700 of the present invention includes an input differential stage 710, again stage 720, and an output stage 760. Input differential stage 710 isconfigured by pnp bipolar junction transistors 711 and 712, forming atransconductance amplifier. Gain stage 720, electrically coupled betweeninput differential stage 720 and output stage 760, is operable toprovide the gain for low offset error amplifier 700 and to prevent theoffset currents from reflecting back to input differential stage 710. Inorder to achieve such results, gain stage 720 further includes a cascodecircuit 730 and an operational amplifier (op-amp) circuit 740. Cascodecircuit 730, configured by cascode bipolar transistors 732 and 733, iselectrically coupled to the input differential stage 710 to form adifferential folded cascode circuit. Op-amp circuit 740 includes adifferential pair 747-748, a plurality of curror mirrors 741-742,743-744, and 745-746. Op-amp circuit 740 is electrically coupled inseries to provide common mode biases to the differential folded cascodecircuit so that offset current is not reflected to input differentialstage 710.

In operation, in order to achieve very low input offset voltage, erroramplifier 700 uses a common-mode feedback scheme to hold the collectorvoltage of gain block 730, more particularly, transistors 732 and 733,at a constant DC voltage which is 2 times the base emitter voltage(2*V_(BE)) above electrical ground 111. Differential pair 747 and 748 inop-amp block 740 will servo the collectors of transistors 732 and 733 toits base voltage. Therefore, node N_(A) and N_(B) have the samecommon-mode DC voltage. As such, error amplifier 700 minimizes theinduced DC offset at input stage due to Early Effect of bipolartransistors 732 and 733. In addition, this embodiment of the presentinvention will provide a differential voltage which will be more immunedto noise.

Error amplifier 700 used in low drop-out voltage regulator 600 is thesame as the low input offset voltage amplifier described in the U.S.patent application titled, “Low Input Offset Amplifier and Method ofMaking Same”, by Farhood Moraveji which is filed on the same dateherewith and which is hereby incorporated by reference in its entirety.

Referring now to FIG. 8, a flow chart 800 showing of a method ofreducing undershoot and overshoot errors in analog integrated circuitsis illustrated. Method 800 includes the steps of detecting whether anundershoot or an overshoot error occurs in the output signal using aseparate overshoot correction circuit and undershoot correction circuit,when either an undershoot error or an overshoot error is detected, thengenerating a corrective signal, and using the corrective signal to spurthe analog integrated circuit to react faster to reduce the undershootor overshoot error. Method 800 reduces overshoot and undershoot errorsin the output signal by improving the reaction time (Δt) of the analogintegrated circuit without consuming a large amount of quiescentcurrents. Equivalently, method 800 dynamically adjusts the loopbandwidth of an analog integrated circuit whenever either an undershooterror or an overshoot error is detected. Furthermore, in the occurrenceof overshoot error, creating an electrical path to dissipate excessenergy carried in the overshoot error.

Now referring to step 801, the output signal of an analog integratedsystem is sensed using the undershoot correction circuit and overshootcorrection circuit. The input terminal of overshoot correction circuitis coupled to sense the output voltage of the analog integrated for anyovershoot error. The undershoot correction circuit is used to sense theoutput voltage for any undershoot error. Step 801 can be implementedusing undershoot correction circuit 201 and overshoot correction circuit202 of the present invention. Undershoot correction circuit 201 iscoupled to sense the output voltage (V_(OUT)) of analog integratedsystem 600. A sudden drop in the output voltage level is detected byhigh speed class AB buffer 411. On the other hand, overshoot correctioncircuit 202 senses the output voltage (V_(OUT)) by comparing it with thevoltage at node V_(A) at the gate of NMOS transistor 611. In the normaloperating condition where there is no overshoot, the voltage at nodeV_(A) is greater than the output voltage (V_(OUT)). When load current112 is disconnected, it causes an increase in output voltage (V_(OUT))as shown in waveform 122 in FIG. 1B. At that moment, the output voltage(V_(OUT)) is greater than the voltage at node V_(A). Overshootcorrection circuit 202 detects the change in the voltage relationshipbetween node V_(A) and output terminal 630.

Now referring to step 802, after the output voltage is sensed, determinewhether overshoot error or an undershoot error has been detected. Thatis, determine whether the status quo condition of either the overshootcorrection circuit or the undershoot correction circuit is changed. Step803 is implemented by differential stage circuit 420 of undershootcorrection circuit 201 and differential stage 520 of overshootcorrection circuit 202. In the normal operating condition, differentialstage 420 is a cutoff mode. That is, first NMOS transistor 421 and firstPMOS transistor 431 are in cutoff mode. As soon as there is a voltagedrop at input terminal 432, buffer 411 would detect this voltage drop.The voltage across resistor 425 decreases accordingly. The decrease inthe voltage across resistor 425 would cause first NMOS transistor 421 tobecome conduct and thus sinks the gate of PMOS transistor 431 towardelectrical ground 111, turning on PMOS transistor 431. As a result,output terminal 433 becomes high and corrective signal is sent out.Similarly, the electrical characteristics of differential stage 520 ofovershoot correction circuit 202 are changed when overshoot error isdetected at input terminal 501. In normal operating condition, first pnpbipolar junction transistor 521 is off and second pnp bipolar junctiontransistor 522 is on. When an overshoot error occurs, output voltage(V_(OUT)) increases, causing second NMOS transistor 521 to turn on. As aresult, first pnp bipolar junction transistor 521 turns off. Thus, thestate of overshoot correction circuit 202 is changed upon the occurrenceof an overshoot error.

Now referring to step 803, when there is no change in the output voltage(V_(OUT)), the electrical characteristics of undershoot correctioncircuit 201 and overshoot correction circuit 202 do not change. Step 804continues to step 802.

Referring to step 804, a corrective signal is issued when the electricalcharacteristics of either undershoot correction circuit and overshootcorrection circuit has been changed. Step 805 is implemented by PMOStransistor 431 of undershoot correction circuit 201 and second pnpbipolar junction transistor 522 of overshoot correction circuit 202.More particularly, when the current state of undershoot correctioncircuit 201 is changed, PMOS transistor 431 is turned ON, pulling outputterminal 433 to supply voltage (V_(CC)) 110 and thus sending acorrective signal to error amplifier 601. In case of overshootcorrection circuit 202, the increase in the base current of first pnpbipolar junction transistor 521 caused by a decrease in the base currentof second npn bipolar junction transistor 522. As a result, outputterminal 528 sees a voltage drop across resistor 527.

Finally, referring to step 805, corrective signal causes analogintegrated circuit to react promptly to undershoot error and overshooterror. Thus, step 806 improves the reaction time Δt of the analogintegrated circuit, and thus reducing the amplitude of undershoot errorand overshoot error. Step 805 is implemented by the signal at outputterminal 433 of undershoot correction circuit 201. Corrective signalcauses error amplifier 601 to produce correcting current at the gate ofpass transistor 611, thus compensating for the undershoot problem. Onthe other hand, the corrective signal causes pass transistor 611 to turnoff, thus eliminating the overshoot problem. Furthermore, in oneembodiment of the present invention, method 800 also includes a step ofproviding an electrical discharge path for output signal so thatovershoot error can be dumped. This step is implemented by theelectrical path formed by NMOS transistor 511.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described. It should beunderstood, of course, the foregoing disclosure relates only to apreferred embodiment (or embodiments) of the invention and that numerousmodifications may be made therein without departing from the spirit andthe scope of the invention as set forth in the appended claims. Variousmodifications are contemplated and they obviously will be resorted to bythose skilled in the art without departing from the spirit and the scopeof the invention as hereinafter defined by the appended claims as only apreferred embodiment(s) thereof has been disclosed.

1. An error correction circuit for an analog integrated circuit havingan input terminal and an output terminal, comprising: an undershootcorrection circuit electrically coupled to detect an undershoot error insaid analog integrated circuit, said undershoot correction circuitoperable to improve the reaction time of said analog integrated circuitonly when said undershoot error is detected at said output terminal; andan overshoot correction circuit electrically coupled to detect anovershoot error in said analog integrated circuit, said overshootcorrection circuit operable to improve the reaction time of said analogintegrated circuit only when said overshoot error is detected at saidoutput terminal.
 2. The error correction circuit of claim 1 wherein saidovershoot correction circuit is operable to create an electrical path tocompensate said overshoot error as soon as said overshoot error isdetected.
 3. The error correction circuit of claim 1 wherein saidovershot correction circuit is activated by changing its electricalcharacteristics from a high impedance state to a low impedance state,and wherein said undershoot correction circuit is activated by changingits electrical characteristics from a high impedance state to a lowimpedance state.
 4. The error correction circuit of claim 1 wherein saidundershoot correction circuit further comprises: an input stage circuitincluding an input terminal electrically coupled to detect saidundershoot error at said output terminal of said analog integratedcircuit; a differential stage circuit electrically coupled to said inputterminal; and an output stage circuit electrically coupled to saiddifferential stage circuit and to said analog integrated circuit, assoon as said undershoot error is detected, said differential stagecircuit and said output stage circuit are activated to generate acorrective signal that improves the reaction time of said analogintegrated circuit.
 5. The error correction circuit of claim 4 whereinsaid input stage comprises a high speed class AB buffer electricallycoupled to a capacitor.
 6. The error correction circuit of claim 4wherein said differential stage circuit further comprises: a first NMOStransistor electrically coupled to a second NMOS transistor, the gate ofsaid first NMOS transistor is electrically connected to the gate of saidsecond NMOS transistor, the source of said first NMOS transistorelectrically coupled to the first terminal of a first resister and tosaid input stage circuit, the second terminal of said first resistor iselectrically coupled to an electrical ground, the source of said secondNMOS transistor electrically coupled to the first terminal of a secondresistor, the second terminals of said second resistor electricallycoupled to said electrical ground; and a biasing circuitry electricallycoupled to said first NMOS transistor and said second NMOS transistor,wherein said biasing circuitry is operable to bias said first NMOStransistor and said second NMOS transistor, the collectors of said firstNMOS transistor and said second NMOS transistor electrically coupled tosaid biasing circuitry and said output stage circuit.
 7. The errorcorrection circuit of claim 6 wherein said biasing circuitry comprises afirst current source comprising a first PMOS transistor and a secondPMOS transistor, the gate of said first PMOS transistor electricallycoupled to the gate of said second PMOS transistor, the drain of saidfirst PMOS transistor electrically coupled to the drain of said firstNMOS transistor and to said output stage circuit, the drain of saidsecond PMOS transistor electrically coupled the drain of said secondNMOS transistor and to its gate, the source of said first PMOStransistor electrically coupled to the source of said second NMOStransistor and to a supply voltage; and a second current source having afirst terminal and a second terminal, said first terminal electricallycoupled to said supply voltage; a third NMOS transistor having the drainelectrically coupled to a second terminal of said second current source,the gate of said third NMOS transistor electrically coupled to the drainand to the gates of said first NMOS transistor and said second NMOStransistor, the source of said third NMOS transistor electricallycoupled to the first terminal of a third resistor, the second terminalof said third resistor electrically coupled to said electrical ground.8. The error correction circuit of claim 7 wherein said output stagecircuit is a PMOS transistor, the gate of said PMOS transistorelectrically coupled to the drain of said first NMOS transistor, thedrain of said PMOS transistor forming said output terminal andelectrically coupled to said analog integrated circuit, and the sourceelectrically coupled to said supply voltage.
 9. The error correctioncircuit of claim 1 wherein said overshoot correction circuit furthercomprises: a differential stage circuit electrically coupled to saidoutput terminal of said analog integrated circuit; and an output stagecircuit electrically coupled to said differential stage circuit and tosaid analog integrated circuit.
 10. The error correction circuit ofclaim 9 wherein said differential stage circuit further comprising: afirst pnp bipolar junction transistor electrically coupled to a secondpnp bipolar junction transistor, the base of said first pnp bipolarjunction transistor is electrically connected to the base of said secondpnp bipolar junction transistor, the emitter of said first pnp bipolarjunction transistor electrically coupled to the first terminal of aresistor, the second terminal of said resistor electrically coupled tosaid analog integrated circuit, the emitter of said second pnp bipolarjunction transistor electrically forming said input terminal and coupledto receive said output signal of said analog integrated circuit; andbiasing circuitry electrically coupled to said first pnp bipolarjunction transistor and said second pnp bipolar junction transistor,said biasing circuitry operable to bias said differential stage, thecollectors of said first pnp bipolar junction transistor and said secondpnp bipolar junction transistor electrically coupled to said biasingcircuitry.
 11. The error correction circuit of claim 10 wherein saidbiasing circuitry comprises: a first current source comprising a firstnpn bipolar junction transistor and a second npn bipolar junctiontransistor, the base of said first npn bipolar junction transistorelectrically coupled to the base of said second npn bipolar junctiontransistor, the collector of said first npn bipolar junction transistorelectrically coupled to its base and to the collector of said first pnptransistor, the collector of said second npn bipolar junction transistorelectrically coupled to the collector of said second pnp bipolarjunction transistor and to said output stage circuit, the emitter ofsaid first npn bipolar junction transistor electrically coupled to theemitter of second npn transistor and to an electrical ground; a secondcurrent source having a first terminal and a second terminal, the firstterminal electrically coupled to said electrical ground; and a third pnpbipolar junction transistor, the collector of said third pnp transistorelectrically coupled to its base and to the bases of first pnp, secondpnp bipolar junction transistors, and to the second terminal of saidsecond current source, the emitter of said third pnp bipolar junctiontransistor electrically coupled to the first terminal of said resistorand to the emitter of said first pnp bipolar junction transistor. 12.The error correction circuit of claim 11 wherein said output stagecircuit comprises an NMOS transistor, the gate of said NMOS transistorelectrically coupled to the collector of said second pnp bipolarjunction transistor, the drain of said NMOS transistor electricallycoupled to said analog integrated circuit and to the emitter of saidsecond pnp bipolar junction transistor, the source of said NMOStransistor electrically coupled to said electrical ground.
 13. Themethod of correcting overshoot and undershoot problems in an analogintegrated circuit comprising: sensing for either undershoot orovershoot errors in said analog integrated circuit using an errorcorrection circuit that includes an undershoot correction circuit and anovershoot correction circuit; whenever either said undershoot error orsaid overshoot error is detected, generating a corrective signal,otherwise, continuing said sensing for either undershoot or overshooterror; and using said corrective signal to cause said analog integratedcircuit to react immediately so as to substantially reduce saidundershoot and overshoot errors.
 14. The method of claim 13 furthercomprising the step of biasing said undershoot correction circuit sothat said undershoot correction circuit operates in a cutoff state andthat said undershoot correction circuit is turned on whenever saidundershoot error is detected.
 15. The method of claim 13 furthercomprising the step of biasing said overshoot correction circuit so thatsaid overshoot correction circuit operates in a cutoff state and thatsaid overshoot correction circuit is turned on whenever said overshooterror is detected.
 16. The method of claim 13 further comprising thestep of coupling said undershoot correction circuit and said overshootcorrection circuit to said analog integrated circuit.
 17. The method ofclaim 13 further comprising the step of creating an electrical path todissipate said overshoot error when said overshoot error is detected.18. A low drop-out (LDO) voltage regulator circuit, comprising: an erroramplifier comprising a first input terminal, a second input terminal,and an output terminal; a pass device electrically coupled in series tosaid output terminal of said error amplifier for receiving a correctivesignal from said error amplifier; and an error correction circuitfurther comprising an undershoot correction circuit electrically coupledto detect an undershoot error in an output signal of said low drop-outvoltage regulator circuit, said undershoot correction circuit operableto substantially reduce said undershoot error by improving the reactiontime of said low drop-out voltage regulator circuit, wherein as soon assaid undershoot error is detected, said undershoot correction circuit isactivated and operable to immediately cause said low drop-out voltageregulator circuit to start compensating for said output signal; and anovershoot correction circuit electrically coupled to detect an overshooterror in said output signal, said overshoot correction circuit operableto substantially reduce said overshoot error by improving the reactiontime of said low drop-out voltage regulator circuit, wherein as soon assaid overshoot error is detected, said undershoot correction circuit isactivated and operable to immediately prevent said low drop-out voltageregulator circuit from compensating for said output signal.
 19. The lowdrop-out voltage regulator circuit of claim 18 wherein said undershootcorrection circuit is in a cutoff state when said undershoot error isnot detected and when said undershoot error is detected, said undershootcorrection circuit is activated and operable to cause said low drop-outvoltage regulator circuit to start compensating for said output signalso that said undershoot error is substantially reduced, said undershooterror correction further comprises: an input stage circuit including aninput terminal electrically coupled to detect said undershoot error; adifferential stage circuit electrically coupled to said input terminal;and an output stage circuit electrically coupled to said differentialstage circuit and to said error amplifier.
 20. The low drop-out voltageregulator circuit of claim 18 wherein said overshoot correction circuitis in a cutoff state when said overshoot error is not detected andwherein when said overshoot error is detected, said undershootcorrection circuit is turned on and operable to cause said erroramplifier to stop compensating for said output signal, said overshootcorrection circuit further comprising: a differential stage circuitelectrically coupled to detect said overshoot error in said low drop-outvoltage regulator circuit; and an output stage circuit electricallycoupled to said differential stage and to said error amplifier.
 21. Thelow drop-out voltage regulator circuit of claim 18 wherein said passdevice is a power transistor further comprises a gate terminal, a sourceterminal, and a drain terminal, said gate terminal electrically coupledto said output terminal of said error amplifier and said drain terminalelectrically coupled to receive said corrective signal from said erroramplifier.
 22. The low drop-out voltage regulator circuit of claim 18further comprising a divider network electrically coupled to said outputterminal of said error amplifier and said pass device.
 23. The lowdrop-out voltage regulator circuit of claim 18 wherein said erroramplifier is a low input offset error amplifier that further comprises:an input differential stage; a gain stage electrically coupled to saidinput differential stage, said gain stage further comprises a cascodecircuit electrically coupled to an operational amplifier (op-amp)circuit; and an output stage electrically coupled to said op-ampcircuit.
 24. The low drop-out voltage regulator circuit of claim 23wherein said op-amp circuit further comprises a differential pairelectrically coupled to a plurality of current mirrors, and wherein saidop-amp circuit and said cascode circuit forms a common mode feedbackloop and wherein said op-amp circuit provides a common mode bias to saidcascode circuit.
 25. The low drop-out voltage regulator circuit of claim18 wherein said cascode circuit further comprises a first cascodetransistor and a second cascode transistor electrically coupled togetherand to said input differential stage to form a folded cascodedifferential pair,